Patch: passive serialization

[Andrew Doran <ad%netbsd.org@localhost>]

This patch implements an algorithm similar to read-copy-update *. It is
covered by the lapsed US patent 4809168 ** and is a good technique for
making some table or lookup-type operations lockless. See ras_lookup()
for a good application.

        http://www.netbsd.org/~ad/cleanout/psync.diff
        http://www.google.com/patents?id=E14WAAAAEBAJ&dq=4809168

Notes:

- I belive that I have accurately implemented the algorithm described in
  the patent.

- The patch is against an earlier version of NetBSD and may not compile.

- It has not been tested and so should be considered as illustrating the
  concept only. The code does seem to be ok.

- No memory barriers are required on the reader side as issuing a cross-call
  makes CPUs in an MP system behave as if they were strongly ordered and
  without store buffers, in respect to certain types and method of
  operation. Proof not provided!

- Readers must do this, and must not block while in the critical section:

        kpreempt_disable();
        /* do lookup operation */
        kpreempt_enable();

- Writers do:

        mutex_enter(&update_lock);
        /* make updates, install new data items for readers. */
        /* publish the updates. */
        xc_sync(&sync_ojbect);
        /* now safe to destroy any old data items. */
        mutex_exit(&update_lock);

- It could be modified to use synchronization points other than context
  switch. For example, a low-priority soft interrupt on all CPUs, triggered
  via IPI. In that case the reader would do 'splsoftclock()+splx()'. The
  patent would have to be inspected to see if this is covered.

Andrew

* http://en.wikipedia.org/wiki/Read-copy-update
** this means it is 'patent-free' in the United States